Upon encountering objects that they recognize as foreign, a primary response of the cells (hemocytes) of the insect immune system is their aggregation in multiple layers on the surfaces of the foreign object in a process known as encapsulation. As the initiation of aggregation and encapsulation occurs, hemocytes switch from their usual nonadherent state to an adherent state and begin expressing new adhesion proteins that orchestrate the encapsulation process. Many pathogens and parasites, however, are able to circumvent this host defense of insects by either preventing or suppressing the expression of adhesion receptors. Although a great variety of adhesion receptors are known to be involved in the interactions of cells in the mammnalian immune system, no comparable receptors involved in regulating encapsulation are yet known for insects. To establish the role of adhesion receptors in the insect immune response, this study will proceed by several routes using the large, well-studied moth Manduca sexta. Three groups of proteins implicated in plasmatocyte adhesion and encapsulation will be investigated. (1) The function of the granular cell-specific protein lacunin in adhesion of plasmatocytes will be tested. This multidomain protein is present in capsules at the interface between the foreign surface and the adherent plasmatocytes. Experiments will test the role of the thrombospondin domains of lacunin in plasmatocyte adhesion. (2) The function of insect cell adhesion proteins from the immunoglobulin superfamily in hemocyte encapsulation will be investigated. Recombinant proteins representing the putative adhesive domains of three surface proteins having iminunoglobulin-like domains as well as antibodies generated to these surface proteins will be assayed for their effects on aggregation and encapsulation of hemocytes. (3) Two plasmatocyte integral membrane proteins, which function in encapsulation, will be characterized. Monoclonal antibodies against these surface proteins disrupt plasmatocyte aggregation. and encapsulation. These monoclonal antibodies were used to affinity purify the proteins for partial sequencing. With information now available on the partial sequences of several peptides from each protein, probes will be generated for screening of a hemocyte cDNA library and obtaining their complete sequences, which should reveal additional features of these adhesion receptors. Understanding the molecular underpinnings of the insect immune response will clearly help in understanding the fate of pathogens in the blood of insect vectors of human diseases.